This invention relates generally to instrumentation, tools and techniques associated with spinal fixation and, in particular, to apparatus and methods facilitating spinal correction in multiple dimensions.
The human spine exhibits some degree of curvature at different levels to facilitate normal physiologic function. Correction may be required when this curvature deviates substantially. A common problem is lateral deviation of the spine, commonly termed scoliosis.
Spinal deformity occurs when a patient has abnormal frontal or sagittal plane alignment. At the same time, the cervical and lumbar spine exhibit lordosis, while the thoracic spine has kyphosis. Thus, when performing spinal fusion, surgeons may be required to preserve or restore both front plane and sagittal alignment while taking lordosis and kyphosis into account.
As discussed in U.S. Pat. No. 5,540,689, the first successful internal fixation method for surgically treating scoliosis used the Harrington instrumentation system. According to this technique, a rigid rod with hooks at each end is implanted adjacent the concave side of the scoliotic spine. The spine is manually straightened to a desired extent and a distraction rod is used to maintain the correction by exerting vertical forces at each end. The rod commonly has a ratcheted end over which hooks are slidably mounted and locked in place. To accommodate lordosis, a compression rod is sometimes placed on the convex side of the scoliotic spine.
The Harrington instrumentation system has been used successfully for some time, but because the distraction rod is fixed to the spine in only two places, failure at either end causes the entire system to fail. Another deficiency with existing mechanisms and approaches is that the single rod used to correct the defects must be contoured to fit various attachment sites. In patients having compound spinal deformity, this may be extremely difficult. A further problem is that the contoured rod frequently limits further correction of certain types of deformities. That is, once the rod is in position, further correction of the deformity is difficult, since existing systems tend to limit incremental alignment procedures.
An alternative treatment has since evolved which takes advantage of segmented fixation. According to this method, a rod is fixed to the spine at multiple points by means of sublaminar wires which run underneath the lamina of the vertebra an around the rod. The use of multiple fixation sites enhances stability and reduces the need for additional post-operative bracing.
Sublaminar fixation utilizing current devices has two primary weaknesses, however. First, the wires are simply wrapped around the rod, resulting in a rod to cable junction which is not rigid. Second, the thin wires can cut in some instances right through the lamina.
U.S. Pat. No. 6,019,759 uses multiple longitudinal members with at plates that attach using hooks or screws. However, the plates are stacked on top of one another at each attachment site, resulting in an overall structure that tends to be quite thick. Systems having a high sagittal profile are often thick enough to be felt through the skin. Additionally, the teaching of the ""759 patent do not allow for easy correction or preservation of sagittal alignment.
The need remains, therefore, for a system and method that allows incremental correction of spinal defects, ideally in all three dimensions.
This invention resides in spinal alignment apparatus, including implantable components, instrumentation, and methods of use. In broad and general terms, the preferred embodiment includes bodies which connect to the vertebra to be aligned, and elongated elements that connect to the bodies. The elements are preferably adjustable relative to the bodies in multiple dimensions, with locking mechanisms that allow the alignment to proceed in an orderly fashion until a desired degree of correction is achieved.
Each rigid, elongated element has at least one end terminating in the first portion of the lockable coupling mechanism. The vertebral connector bodies each include a feature for attaching the body to a respective vertebrae, and the second portion of the lockable coupling mechanism. This arrangement permits the elongated elements to be adjusted in multiple dimensions relative to a given connector body prior to being lockingly coupled thereto.
The feature for attaching the body to its respective vertebrae may include a pedicle screw or, alternatively, a shape such as a hook adapted for sublaminar engagement. The elongated elements may also preferably include a length adjustment mechanism, such as a telescoping or threaded section, to provide a desired length in conjunction with a desired degree of alignment.
Various coupling mechanisms are disclosed to provide multiple degrees of freedom prior to fixation. In the preferred embodiment, the mechanism includes a fixed or adjustable-length rod having ball-shaped ends coupled to a vertebral connector providing multiple degrees of freedom before being locked into position once a desired orientation is achieved.